1. Field of the Invention
The present invention relates to a process for preparing hexafluoropropylene(CF3CFxe2x95x90CF2, HFP) from the pyrolysis of trifluoromethane(CHF3, R23) and tetrafluoroethylene(C2F4, TFE) and more particularly, to the process for preparing hexafluoropropylene from the pyrolysis of an admixture of R23 and TFE mixed in an appropriate molar ratio at below 900xc2x0 C. which is lower than the conventional reaction temperature and longer residence time, after investigating the pyrolysis reaction of R23 and TFE by the computer simulation. The process for preparing HFP is performed by carefully controlling reaction temperature with heat balance resulted from an endothermic pyrolysis of R23 and an exothermic dimerization of TFE to prevent from carbon formation, recycling unreacted R23 and TFE in the product separated and purified from distillation column, adding fresh R23 additionally to keep an appropriate molar ratio of R23 and TFE, to improve a total yield of HFP and to minimize heat supply from outside.
2. Description of the Prior Art
As a monomer for preparing copolymers of fluorinated resins, the demand of HFP has been increased along with TFE. Conventional methods for preparing HFP are pyrolysis of difluorochloromethane(CHClF2, R22) (EP Patent No. 0,287,219 (1988) and U.S. Pat. No. 4,849,554 (1989)), pyrolysis of TFE and octafluorocyclobuthane(C4F8, RC318) (U.S. Pat. No. 3,446,858 (1969)), pyrolysis of polytetrafluoroethylene(PTFE) (U.S. Pat. No. 2,759,983 (1956)), and pyrolysis of R23 (U.S. Pat. No. 3,009,966 (1961)).
The process of preparing HFP by the pyrolysis of R22 has low selectivity to HFP due to high selectivity of TFE formation and further, it is difficult to separate pure HFP from an azeotropic mixture of R22 and HFP. The pyrolysis of TFE to produce HFP suffers from a low selectivity to HFP because it mainly produces RC318. The pyrolysis of PTFE has complicate process wherein TFE is first prepared by pyrolysis of R22 and then polymerized to produce PTFE, followed by the pyrolysis thereof. And further, even if the selectivity to HFP is higher than that from the pyrolysis of R22 or TFE, it is an undesirable method because of expensive unit price.
Another method to produce TFE and HFP has been reported by Hauptschein and Fainberg (U.S. Pat. No. 3,009,966 (1961)). HFP is prepared by the pyrolysis of R23 at a temperature range of 700 to 1,500xc2x0 C. and a residence time of 0.5 to 0.001 sec. Unreacted R23 and TFE, after separating form the product, are recycled to increase the selectivity to HFP. However, it suffers from the disadvantage of carbon formation as a side-reaction hindering the original pyrolysis, because this pyrolysis has to be performed at a high temperature above 900xc2x0 C. This reaction requires continuous supply of heat from outside, which is expensive, and the temperature range is too broad during the reaction because it is difficult to control the reaction temperature. Politanskii et al. has reported that R23 is decomposed to carbene(:CF2) and HF (S. F. Poltanskii and V. U. Shevchuk, Kinetika I Kataliz, 9, 496 (1968)) and the pyrolysis reaction of R23 is an endothermic reaction, which requires continuous supply of heat from outside to keep the temperature above 900xc2x0 C. On the other hand, the dimerization of TFE is an exothermic reaction which requires continuous removal of heat. Therefore, when the pyrolysis of R23 is performed at above 1000xc2x0 C., predominant byproduct TFE undergoes drastic dimerization and thereby, generates heat. As a result of this, the reaction temperature rapidly increases and it can be almost impossible to control the reaction temperature. Hauptschein and Fainberg have performed the pyrolysis reaction in the broad temperature range of from 700 to 1500xc2x0 C. because of the drawback of controlling the reaction temperature resulted from an endothermic pyrolysis of R23 and an exothermic dimerization of TFE byproduct. However, when the reaction temperature is above 1000xc2x0 C. in the pyrolysis of R23 to produce HFP, carbon formation reaction is predominately proceeded. So it is difficult to control the reaction temperature as well as to prevent from carbon formation.
Therefore, an object of the present invention is to provide an improved process for preparing HFP from the pyrolysis of an admixture of R23 and TFE mixed in an appropriate molar ratio, which is derived from the computer simulation of the pyrolysis of R23 and TFE for estimating an appropriate total heat calculated from an endothermic pyrolysis of R23 and an exothermic dimerization of TFE, to stabilize the reaction temperature.
Another object of the present invention is to perform the pyrolysis of said admixture at a temperature below 900xc2x0 C. and a residence time of 0.1 to 5 to improve in controlling the reaction temperature and to prevent from carbon formation to produce HFP.
Another object of the present invention is to recycle unreacted R23 and TFE separated and purified from the product and to add fresh R23 for controlling an appropriate molar ratio of R23/TFE in order to obtain a high yield of HFP and to minimize heat supply from outside.
And further object of the present invention is to maximize the production of HFP from the pyrolysis of R23 and TFE for industrial scale based on the result of small scale obtained from the above conditions.
In the present invention, it is characterized in that hexafluoropropylene is prepared from the pyrolysis of the mixture of trifluoromethane and tetrafluoroethylene mixed in the molar ratio of R23/TFE of 0.25 to 10 under the pyrolysis reaction conditions of R23 such as a reaction temperature of 750 to 950xc2x0 C. and a residence time of 0.1 to 5 sec.
The present invention is described in more detail as set forth hereunder.
The present invention requires the computer simulation to investigate the pyrolysis of R23 for more efficient performance of the reaction. The pyrolysis reaction of R23 is an endothermic reaction, which requires continuous supply of heat from outside even at the temperature above 900xc2x0 C., while the dimerization of TFE is an extreme exothermic reaction, which requires continuous removal of heat.
Therefore, the present invention requires the process to estimate the reaction conditions through the computer simulation for the adiabatic reaction condition, because the reaction temperature can be controlled by supplying both R23 and TFE in an appropriate ratio to a reactor. FIG. 1 shows the estimated result for the effect of temperature verses residence time for different molar ratio of R23 and TFE. As shown in FIG. 1, it indicates that the reaction temperature is rapidly increased at above 850xc2x0 C., where the pyrolysis of R23 occurs, due to dimerization of TFE which is an exothermic reaction and thus, it is almost impossible to control the reaction temperature.
Based on the result of said computer simulation, it can be expected to perform the pyrolysis reaction of an a mixture of R23 of 50 to 80 mole % and TFE of 20 to 50 mole % to produce HFP by stabilizing the reaction temperature.
Therefore, the present invention provides an effective process for preparing HFP based on the best reaction conditions estimated through the computer simulation for the pyrolysis of an admixture of R23 and TFE.
FIG. 2 shows the pyrolysis system with the pyrolysis of R23 and the purification for industrial applications. The present invention is characterized by the apparatus of FIG. 2 for the pyrolysis of R23 and TFE. It is important to apply an appropriate molar ratio of R23 and TFE to the reactor for the pyrolysis of an admixture of R23 and TFE. The present invention is also to provide maximum production of HFP by recycling unreacted R23 and TFE and stabilizing the reaction temperature between an endothermic pyrolysis of R23 and an exothermic dimerization of TFE.
The pyrolysis of the present invention is performed at a reaction temperature of 750 to 950xc2x0 C., preferably 800 to 900xc2x0 C. and a residence time of 0.1 to 5 sec, preferably 0.1 to 2 sec. If the reaction temperature is below 750xc2x0 C., the production of HFP is low. On the other hand, if it is above 950xc2x0 C., the formation of TFE increases and activates the dimerization of TFE and thus, it can be difficult to control the reaction temperature so that the production of HFP becomes low.
The preferable molar ratio of R23/TFE is in the range of 0.25 to 10, more preferably 1 to 4. If it is lower than 0.25, it activates the dimerization of TFE so that the reaction temperature can rapidly increase. If it is higher than 10, it requires additional heat supply from outside for the pyrolysis of R23.
The process of the present invention maximizes the production of HFP under the reaction conditions described above by using the apparatus of FIG. 2.
The apparatus for the pyrolysis of R23 and purification of IFP is comprised with a R23 cylinder (1), a TFE cylinder (2), a mass flow controller (3), a pre-heater (4), a reactor (5), process water (6), a quenching column (7), condensing water (8), an acid reservoir (9), an acid washing column (10), a diaphram gas compressor (11), a dryer (12), a product reservoir (13), the first distillation column (14), a R23/TFE mixture reservoir (15), the second distillation column (16), a HFP tank (17), a 6-port sampling valve (18), a gas chromatograph (19), and a personal computer (20). Said reactor (5) is made of Inconel metal and is tube-type having an outer diameter of xc2xe inches and a length of 1.5 m.
HFP in the present invention is prepared by, passing through the pyrolysis and purification systems in the sequence of the mass flow controller (3) of R23 or an admixture of R23 and TFE, the pre-heater (4), the reactor (5), the quenching column (7), the acid washing column (10), the diaphram gas compressor (11), the dryer (12), the distillation columns (14, 16), and the recycle circuit. Unreacted R23 and TFE are separated from the distillation columns (14, 16) and then stored in the R23/TFE mixture reservoir (15). This separated mixture of R23 and TFE is supplied to the reactor (5) and fresh R23 can be added to the reactor (5) to keep an appropriate molar ratio. This mixture is then performed for the pyrolysis reaction. As a result, this process maximizes the production of HFP and minimizes loss of heat by controlling the reaction temperature. The quenching column (7) is placed at the exit of the reactor and thereby prevents from the formation of solid polymer by quenching with water.
HF produced during the reaction can be removed by passing through the quenching column (7), dissolving in water and then neutralizing at the acid reservoir (9). And again, acid removed product is passed in the sequence of the acid washing column (10) to remove remained HF, the diaphram gas compressor (11), the dryer (12), a reboiler (not shown in FIG. 2) connected to the first distillation column (14). Unreacted R23 and TFE after the pyrolysis of R23 are separated at the first distillation column (14) and stored at the R23/TFE mixture reservoir (15). The compounds having high boiling point are distilled again at the second distillation column (16) to get the desired product with high purity. The product, HFP, is analyzed by gas chromatograph (5890 series Il, Hewllet Packard Inc.) (19) equipped with a poraplot Q capillary column (0.32 mm(OD)xc3x9725 m(L)). The gas chromatography is operated under the conditions of a GC detector temperature of 200xc2x0 C., an injection temperature of 150xc2x0 C., a temperature of column of 90xc2x0 C. and a helium head pressure of 5 psig, The product is identified by GC/MS(HP 5890/5971) equipped with the same column. The apparatus is controlled by the personal computer (20).
The following examples are intended to be illustrative of the present invention and should not be construed as limiting the scope of this invention defined by the appended claims.